Optical system with compensation lens

ABSTRACT

An optical system used in a charged particle beam inspection system. The optical system includes one or more optical lenses, and a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from a first medium to a second medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 16/650,815,filed Mar. 25, 2020, which is a national stage filing under 35 U.S.C. §371 of International Application No. PCT/EP2018/075927, filed on Sep.25, 2018, and published as WO 2019/063529 A1, which claims priority ofU.S. Provisional Application No. 62/564,966 which was filed on Sep. 28,2017, all of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present disclosure generally relates to an optical system in acharged particle beam inspection system and, more specifically, to anoptical system with a compensation lens in a charged particle beaminspection system.

BACKGROUND

A charged particle beam based microscope, such as a scanning electronmicroscope (SEM) or a transmission electron microscope (TEM), capable ofproviding image resolution down to less than a nanometer, serves as apracticable tool for inspecting samples having a feature size that issub-100 nanometers. During operation of the charged particle beam basedmicroscope, a part of the microscope is positioned in a vacuum chamber.

An optical system is usually included in the charged particle beam basedmicroscope to allow visual inspection of a sample, to detect sampleheight, or to illuminate the sample. During operation of the chargedparticle beam based microscope, one or more lenses of the optical systemare position in the vacuum chamber along with the part of the chargedparticle beam based microscope. As the medium of the optical systemchanges, the focal length of the one or more lenses may change,affecting the precision of the optical system.

SUMMARY

According to some embodiments of the disclosure, an optical system usedin a charged particle beam inspection system is provided. The opticalsystem includes one or more optical lenses, and a compensation lensconfigured to compensate a drift of a focal length of a combination ofthe one or more optical lenses from a first medium to a second medium.

According to some embodiments of the disclosure, a method for operatingan optical system used in a charged particle beam inspection system isprovided. The optical system including one or more optical lenses. Themethod includes: providing a compensation lens in the optical system,the compensation lens being configured to compensate a drift of a focallength of a combination of the one or more lenses from a first medium toa second medium; assembling and calibrating the optical system in thefirst medium; removing the compensation lens; and placing the one ormore optical lenses in the second medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments.

FIG. 1 is a schematic diagram of an exemplary charged particle beaminspection system with an optical position detection system, consistentwith some disclosed embodiments.

FIG. 2 is a schematic diagram of an exemplary charged particle beaminspection system with an optical imaging system, consistent with somedisclosed embodiments.

FIG. 3 is a schematic diagram of an exemplary charged particle beaminspection system with an optical illumination system, consistent withsome disclosed embodiments.

FIG. 4 schematically illustrates an exemplary method for configuring acompensation lens for use in an optical system in a charged particlebeam inspection system, consistent with some disclosed embodiments.

FIG. 5 is a flow chart of an exemplary process of operating an opticalsystem in a charged particle beam inspection system, consistent withsome embodiments of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the example embodiments, whichare illustrated in the accompanying drawings. Although the followingembodiments are described in the context of utilizing electron beams,the disclosure is not so limited. Other types of charged particle beamscan be similarly applied.

The disclosed embodiments provide an optical system used in a chargedparticle beam inspection system. The optical system includes one or moreoptical lenses to be disposed under vacuum atmosphere, and acompensation lens configured to compensate a drift of a focal length ofa combination of the one or more optical lenses from atmosphericconditions to vacuum conditions. The optical system and the chargedparticle beam inspection system are assembled and calibrated underatmospheric conditions with the compensation lens. When a part ofcharged particle beam inspection system including the one or moreoptical lenses of the optical system is placed under vacuum conditions,a drift of a focal length of a combination of the one or more opticallenses has been compensated by the compensation lens.

FIG. 1 is a schematic diagram of a charged particle beam inspectionsystem 100 with an optical position detection system 120, consistentwith some disclosed embodiments. Charged particle beam inspection system100 includes a charged particle beam column 110 for irradiating acharged particle beam (e.g., electron beam) on an inspected sample 102,and optical position detection system 120 for measuring and calibratinga height of sample 102 (e.g., a position of sample 102 along aZ-direction illustrated in FIG. 1 ). Optical position detection system120 includes a projection module 130, a receiving module 140, and adetection module 150. Projection module 130 includes an illuminationmodule 132, a grating mask 134, a projection lens 136, and a mirror 138.Projection module 130 is configured to project a light beam to sample102. Receiving module 140 includes a mirror 142 and a receiving lens144. Receiving module 140 is configured to receive a light beamreflected from sample 102. Detection module 150 includes a vacuum window152, a compensation lens 154, and a detector 156. Detection module 150is configured to detect a position of the sample based on the light beamreflected from sample 102. Charged particle beam column 110, projectionmodule 130, receiving module 140, and sample 102 are disposed in aninspection chamber 160.

Grating mask 134 includes a substrate and a grating pattern formed onthe substrate. The grating pattern includes a plurality of gratingmembers arranged periodically along a direction parallel to thesubstrate. A light transmittance of the grating members is lower than alight transmittance of the substrate.

When illumination module 132 emits a light beam downward along aZ-direction perpendicular to a surface of sample 102, the light beamtransmits through grating mask 134 having the grating pattern.Consequently, the light beam carries the grating pattern of grating mask134. The light beam is conjugated by projection lens 136, and reflectedby mirror 138 towards sample 102. At a surface of sample 102, the lightbeam is reflected by sample 102 to form a grating image. The reflectedlight beam transmits towards receiving module 140. In receiving module140, the light beam is reflected by mirror 142 upward along theZ-direction towards receiving lens 144, and is conjugated by receivinglens 144. In detection module 150, the light beam passes through vacuumwindow 152 and compensation lens 154, and reaches detector 156, whichdetects the grating image. Detector 156 compares the detected gratingimage with the grating pattern, and determines the position of sample102 along the Z-direction based on the comparison.

As described above, optical position detection system 120 is used formeasuring and calibrating the height of sample 102, so as to accuratelycontrol the height of sample 102. The accuracy of the height of sampleis essential for controlling focusing condition of charged particle beamcolumn 110, and thus affecting the inspection accuracy of chargedparticle beam inspection system 100.

Charged particle beam inspection system 100 is assembled and calibratedunder atmospheric conditions (e.g., in the air). After charged particlebeam inspection system 100 is assembled and calibrated, inspectionchamber 160 is evacuated to generate vacuum, such that charged particlebeam column 110, projection module 130, receiving module 140, and sample102 are placed under vacuum conditions. Due to the difference betweenthe refractive indices of air and vacuum, the focal length of projectionlens 136 of projection module 130 and the focal length of receiving lens144 of receiving module 140 may drift. The drift in the focal lengthsmay affect the accuracy of the detected height of sample 102.

In order to solve this problem, in the disclosed embodiments,compensation lens 154 is designed to compensate a drift of a focallength of a combined lens group consisting of projection lens 136 andreceiving lens 144 from the atmospheric conditions to the vacuumconditions. In other words, when projection lens 136 and receiving lens144 are placed under atmospheric conditions, compensation lens 154 canemulate an effect where projection lens 136 and receiving lens 144 areplaced under vacuum conditions. A method for configuring compensationlens 154 will be described with references to FIG. 4 .

When charged particle beam inspection system 100 including opticalposition detection system 120 is assembled and calibrated underatmospheric conditions, compensation lens 154 is placed in detectionmodule 150 to emulate an effect when projection lens 136 and receivinglens 144 are placed under vacuum conditions. Therefore, when chargedparticle beam inspection system 100 is assembled and calibrated withcompensation lens 154 under atmospheric conditions, charged particlebeam inspection system 100 is in an effect of being calibrated undervacuum conditions. After charged particle beam inspection system 100 isassembled and calibrated under the atmospheric conditions, compensationlens 154 is removed from detection module 150, and inspection chamber160 is evacuated to place charged particle beam column 110, projectionmodule 130, receiving module 140, and sample 102 under vacuumconditions. Because charged particle beam inspection system 100 has beencalibrated with compensation lens 154, a drift of a focal length of thecombined lens group consisting of projection lens 136 and receiving lens144 has been corrected by compensation lens 154. As a result, the heightof sample 102 can be accurately detected by optical position detectionsystem 120.

FIG. 2 is a schematic diagram of a charged particle beam inspectionsystem 200 with an optical imaging system 220, consistent with somedisclosed embodiments. Charged particle beam inspection system 200includes a charged particle beam column 210 for irradiating a chargedparticle beam (e.g., electron beam) on an inspected sample 202, andoptical imaging system 220 for obtaining an image of sample 202. Opticalimaging system 220 includes an illumination module 230, a detectionmodule 240, and an objective lens 260. Illumination module 230 isconfigured to project a light beam to sample 202. Objective lens 260 isconfigured to conjugate a light beam reflected from sample 202.Detection module 240 includes a detector 242, a compensation lens 244, atube lens 246, a beam splitter 248, and a vacuum window 250. Detectionmodule 240 is configured to detect an image of sample 202 based on thelight beam reflected from sample 202. Charged particle beam column 210,objective lens 260, and sample 202 are disposed in an inspection chamber270.

When illumination module 230 emits a light beam along an X-directionparallel to a surface of sample 202, part of the light beam is reflectedby beam splitter 248 downwards along a Z-direction perpendicular to thesurface of sample 202. The light beam is conjugated by objective lens260 onto a surface of sample 202. At the sample surface, the light beamis reflected by sample 202 to form an image of sample 202. The reflectedlight beam is conjugated by objective lens 260 towards detection module240. In detection module 240, the light beam passes through vacuumwindow 250 and beam splitter 248. Tube lens 246 magnifies the image ofsample 202 formed by the light beam. The light beam then passes throughcompensation lens 244, and reaches detector 242, which detects the imageof sample 202.

Charged particle beam inspection system 200 is assembled and calibratedunder atmospheric conditions (e.g., in the air). When charged particlebeam inspection system 200 is calibrated under the atmosphericconditions, the positions of objective lens 260 and sample 202 areadjusted to achieve an optimum imaging condition (e.g., the light beamreflected by sample 202 is focused on detector 242). After chargedparticle beam inspection system 200 is assembled and calibrated,inspection chamber 270 is evacuated to generate vacuum, such thatcharged particle beam column 210, objective lens 260, and sample 202 areplaced under a vacuum conditions. Due to the difference between therefractive indices of air and vacuum, the focal length of objective lens260 may drift. As a result, the positions of objective lens 260 andsample 202 that were previously adjusted in the atmospheric conditionsmay no longer achieve the optimum imaging condition.

In order to solve this problem, in the disclosed embodiments,compensation lens 244 is designed to compensate a drift of a focallength of objective lens 260 from the atmospheric conditions to thevacuum conditions. In other words, when objective lens 260 is placedunder atmospheric conditions, compensation lens 244 can emulate aneffect where objective lens 260 is placed under vacuum conditions. Whencharged particle beam inspection system 200 is assembled and calibratedunder atmospheric conditions, compensation lens 244 is placed indetection module 240 to emulate an effect where objective lens 260 isplaced under vacuum conditions. Therefore, when charged particle beaminspection system 200 is assembled and calibrated with compensation lens244 under atmospheric conditions, charged particle beam inspectionsystem 200 is in an effect of being calibrated under vacuum conditions.After charged particle beam inspection system 200 is assembled andcalibrated under atmospheric conditions, compensation lens 244 isremoved from detection module 240, and inspection chamber 270 isevacuated to place charged particle beam column 210, objective lens 260,and sample 202 under vacuum conditions. Because charged particle beaminspection system 200 has been calibrated with compensation lens 244, adrift of a focal length of objective lens 260 has been corrected bycompensation lens 244. As a result, optimum imaging condition can beachieved.

FIG. 3 is a schematic diagram of a charged particle beam inspectionsystem 300 with an optical illumination system 320, consistent with somedisclosed embodiments. Charged particle beam inspection system 300includes a charged particle beam column 310 for irradiating a chargedparticle beam (e.g., electron beam) on an inspected sample 302, andoptical illumination system 320 for obtaining an image of sample 302.Optical illumination system 320 includes an illumination module 330, adetection module 340, an objective lens 360, and a mirror 370.Illumination module 330 is configured to project a light beam to aportion of sample 302 under charged particle beam column 310. Objectivelens 360 is configured to conjugate a light beam scattered from sample302. Detection module 340 includes a detector 342, a tube lens 344, abeam splitter 346, a compensation lens 348, and a vacuum window 350.Detection module 340 is configured to detect an image of the portion ofsample 302 based on the light beam scattered from sample 302. Chargedparticle beam column 310, objective lens 360, mirror 370, and sample 302are disposed in an inspection chamber 380.

When illumination module 330 emits a light beam along an X-directionparallel to a surface of sample 302, part of the light beam is reflectedby beam splitter 346 downwards along a Z-direction perpendicular to thesurface of sample 302. The light beam passes through compensation lens348 and vacuum window 350, and is conjugated by objective lens 360. Theconjugated light beam is reflected by mirror 370 onto a portion ofsample 302 directly below charged particle beam column 310. The lightbeam is reflected by sample 302 to form an image of sample 302. Thereflected light beam is further reflected by mirror 370 upwards alongthe Z-direction. The reflected light beam is conjugated by objectivelens 360 towards detection module 340. In detection module 340, thelight beam passes through vacuum window 350, compensation lens 348, andbeam splitter 346. Tube lens 344 magnifies the image of sample 302formed by the light beam. The light beam then reaches detector 342,which detects the image of sample 302.

Charged particle beam inspection system 300 is assembled and calibratedunder atmospheric conditions (e.g., in the air). When charged particlebeam inspection system 300 is calibrated under the atmosphericconditions, the positions of objective lens 360, mirror 370, and sample302 are adjusted to achieve an optimum imaging condition (e.g., thelight beam reflected by sample 302 is focused on detector 342). Aftercharged particle beam inspection system 300 is assembled and calibrated,inspection chamber 380 is evacuated to generate a vacuum, such thatcharged particle beam column 310, objective lens 360, mirror 370, andsample 302 are placed under a vacuum conditions. Due to the differencebetween the refractive indices of air and vacuum, the focal length ofobjective lens 360 may drift. As a result, the positions of objectivelens 360, mirror 370, and sample 302 that were previously adjusted inthe atmospheric conditions may no longer achieve the optimum imagingcondition.

In order to solve this problem, in the disclosed embodiments,compensation lens 348 is configured to compensate a drift of a focallength of objective lens 360 from the atmospheric conditions to thevacuum conditions. In other words, when objective lens 360 is placedunder atmospheric conditions, compensation lens 348 can emulate aneffect where objective lens 360 is placed under vacuum conditions. Whencharged particle beam inspection system 300 is assembled and calibratedunder the atmospheric conditions, compensation lens 348 is placed indetection module 340 to emulate an effect where objective lens 360 isplaced under vacuum conditions. Therefore, when charged particle beaminspection system 300 is assembled and calibrated with compensation lens348 under atmospheric conditions, charged particle beam inspectionsystem 300 is in an effect of being calibrated under vacuum conditions.After charged particle beam inspection system 300 is assembled andcalibrated under the atmospheric conditions, compensation lens 348 isremoved from detection module 340, and inspection chamber 380 isevacuated to place charged particle beam column 310, objective lens 360,mirror 370, and sample 302 under vacuum conditions. Because chargedparticle beam inspection system 300 has been calibrated withcompensation lens 348, a drift of a focal length of objective lens 360has been corrected by compensation lens 348. As a result, optimumimaging condition can be achieved.

FIG. 4 schematically illustrates a method for configuring a compensationlens for use in an optical system in a charged particle beam inspectionsystem, consistent with some disclosed embodiments. The methodillustrated in FIG. 4 can be applied for configuring, e.g., compensationlens 154 in optical position detection system 120 of FIG. 1 .

As illustrated in FIG. 4 , an optical system 400 includes anillumination source 410, a first lens 420 (e.g., projection lens 136), asample 430 (e.g., sample 102), a second lens 440 (e.g., receiving lens144), a compensation lens 450 (e.g., compensation lens 154), and adetector 460 (e.g., detector 156). First lens 420 and second lens 440constitute a combined lens group that has a focal plane. Ideally,detector 460 is disposed at the focal plane of the combined lens group.Illumination source 410, first lens 420, sample 430, and second lens 440are disposed in an inspection chamber 470 which can be evacuated tocreate a vacuum conditions. Compensation lens 450 and detector 460 aredisposed outside of inspection chamber 470.

When inspection chamber 470 is evacuated to switch the environment ofillumination source 410, first lens 420, sample 430, and second lens 440from air to vacuum, the focal length of the combined lens groupconsisting of first lens 420 and second lens 440 may drift (e.g., theposition of the focal plane of the combined length group may shift in aX-direction) due to a difference between the refractive index of air andthe refractive index of vacuum. The drift of the focal length can berepresented by an image distance change dV, where V is an image distanceof the combined lens group. The image distance change dV can berepresented by dV=dV₁+dV_(M), where dV₁ is a drift of the focal lengthpurely resulted from the change of the refractive index from vacuum toair, and dV_(M) is a drift of the focal length purely resulted fromaxial magnification, and dV_(M)=M²*dU, where M represents a lateralmagnification, and dδ=dU≠0 is the objective side principle plane drift.dU represents a drift of an objective side principle plane of thecombined lens group resulted from the change of the refractive indexfrom vacuum to air. As illustrated in FIG. 4 , P represents an objectiveside principle plane of the combined lens group under the vacuumconditions, P_(a) represents an objective side principle plane of thecombined lens group under the atmospheric conditions, P′ represents animage side principle plane of the combined lens group under the vacuumconditions, P_(a)′ represents an image side principle plane of thecombined lens group under the atmospheric conditions, U represents anobject distance of the combined lens group, dU represents an objectdistance change (i.e., the distance between P and P_(a)) resulted fromthe refractive index change, and δ represents the distance between firstlens 420 and P.

Assuming that first lens 420 has an optical power Φ₁ under atmosphericconditions, and second lens 440 has an optical power Φ₂ underatmospheric conditions. Each one of the optical power Φ₁ and Φ₂ can berepresented by:

$\begin{matrix}{\Phi_{j} = {\frac{n_{gj} - n}{n_{gj} - 1}\Phi_{j0}}} & (1)\end{matrix}$where j=1 or 2, n_(gj) is the glass refractive index of first lens 420or second lens 440, n is the refractive index of air, and 1 is therefractive index of vacuum, and Φ_(j0) is the optical power of firstlens 420 or second lens 440 under vacuum conditions.

A change dΦ₁ of the optical power Φ₁ of first lens 420 or a change dΦ₂of the optical power Φ₂ of second lens 440 induced by the refractiveindex change dn from vacuum to air can be represented by:

$\begin{matrix}{{d\Phi}_{j} = {{- \frac{\Phi_{j0}}{n_{gj} - 1}}{dn}}} & (2)\end{matrix}$where j=1 or 2.

Assuming that a combined lens group consisting of first lens 420 andsecond lens 440 has an optical power Φ under atmospheric conditions, theoptical power Φ can be represented by:

$\begin{matrix}{\Phi = {\Phi_{1} + \Phi_{2} - {\frac{t}{n}\Phi_{1}\Phi_{2}}}} & (3)\end{matrix}$where t is the distance between first lens 420 and second lens 440.

A change dΦ of the optical power Φ of the combined lens group induced bythe refractive index change dn from vacuum to air can be represented by:

$\begin{matrix}{{d\Phi} = {{{d\Phi}_{1} + {d\Phi}_{2} + {{t\Phi}_{1}\Phi_{2}{dn}} - {{t\Phi}_{1}{d\Phi}_{2}} - {{t\Phi}_{2}{d\Phi}_{1}}} = {{\left( {1 - {t\Phi}_{20}} \right){d\Phi}_{1}} + {\left( {1 - {t\Phi}_{10}} \right){d\Phi}_{2}} + {{t\Phi}_{10}\Phi_{20}{dn}}}}} & (4)\end{matrix}$

A relationship between the object distance U of the combined lens groupand the image distance of the V combined lens group can be representedby:

$\begin{matrix}{{\frac{1}{U} + \frac{1}{V}} = \frac{\Phi}{n}} & (5)\end{matrix}$

From Equation (5), the image distance change dV induced by therefractive index change dn from vacuum to air can be represented by:

$\begin{matrix}{{dV} = {{{dV}_{1} + {dV}_{M}} = {{V_{0}^{2}\left( {{\Phi{dn}} - {d\Phi}} \right)} - {\left( \frac{V}{U} \right)^{2}{dU}}}}} & (6)\end{matrix}$

dV₁ and dV_(M) can be respectively represented by:

$\begin{matrix}{{dV}_{1} = {V_{0}^{2}\left\lbrack {{\left( {\Phi_{1} + \Phi_{2} - {2{t\Phi}_{1}\Phi_{2}}} \right){dn}} + {\left( {{t\Phi}_{2} - 1} \right){d\Phi}_{1}} + {\left( {{t\Phi}_{1} - 1} \right){d\Phi}_{2}}} \right\rbrack}} & (7) \\{{dV}_{M} = {{- M^{2}}{dU}}} & (8)\end{matrix}$

Therefore, the image distance change dV induced by the refractive indexchange dn from vacuum to air can be determined based on the opticalpower Φ₁ of first lens 420, the optical power Φ₂ of second lens 440, thedistance t between first lens 420 and second lens 440, and therefractive indices of vacuum and air, based on Equations (1)-(8).

Based on optical theory, the position of the objective side principleplane of the combined lens group can be represented by:

$\begin{matrix}{\delta = {\frac{t}{n}\frac{\Phi_{2}}{\Phi}}} & (9)\end{matrix}$

From Equation (9), the object distance change dU can be represented by:

$\begin{matrix}{{dU} = {{d\delta} = {{{- \frac{{t\Phi}_{20}}{\Phi_{0}}}{dn}} + {t\left( {{\frac{1}{\Phi_{0}}{d\Phi}_{2}} - {\frac{\Phi_{20}}{\Phi_{0}^{2}}{d\Phi}}} \right)}}}} & (10)\end{matrix}$

where Φ₂₀ is the optical power of second lens 440 in vacuum, and Φ₀ isthe optical power of the combined lens group in vacuum.

As illustrated in FIG. 4 , compensation lens 450 is disposed outside ofinspection chamber 470, between second lens 440 and detector 460 of thecombined lens group. An object distance U_(c) of compensation lens 450can be represented by:

$\begin{matrix}{U_{c} = {V_{c} - {dV}}} & (11)\end{matrix}$

where V_(c) represents an image distance of compensation lens 450. UsingEquation (11), the image distance V_(c) of compensation lens 450 can bedetermined based on the object distance U_(c) and the image distancechange dV induced by the refractive index change dn from vacuum to airby using Equation (11).

An optical power Φ_(c) of compensation lens 450 can be represented by:

$\begin{matrix}{\Phi_{c} = {\frac{1}{U_{c}} + \frac{1}{V_{c}}}} & (12)\end{matrix}$

Therefore, the optical power Φ_(c) of compensation lens 450 can bedetermined based on the object distance U_(c) of compensation lens 450,and the image distance V_(c) of compensation lens 450 by using Equation(12). Once the optical power Φ_(c) of compensation lens 450 isdetermined, compensation lens 450 can be formed and used in the opticalsystem to compensate the change dΦ of the optical power Φ of thecombined lens group.

As discussed above, the method described above with respect to FIG. 4can be applied for configuring compensation lens 154 in optical positiondetection system 120 of FIG. 1 . Compensation lens 244 in opticalimaging system 220 of FIG. 2 , or compensation lens 348 in opticalillumination system 320 of FIG. 3 can be configured using a methodsimilar to the method described above with respect to FIG. 4 .

FIG. 5 is a flow chart of a process 500 of operating an optical systemin a charged particle beam inspection system, consistent with someembodiments of the disclosure. The charged particle beam inspectionsystem can be one of charged particle beam inspection systems 100, 200,and 300 respectively illustrated in FIGS. 1-3 .

As illustrated in FIG. 5 , first, the optical system is assembled underatmospheric conditions (step 510). The optical system call be opticalposition detection system 120 illustrated in FIG. 1 , optical imagingsystem 220 of FIG. 2 , or optical illumination system 320. The opticalsystem includes one or more lenses to be disposed under vacuumconditions. A compensation lens is provided in the optical system (step512). The compensation lens is configured to compensate a drift of afocal length of a combination of the one or more lenses from atmosphericconditions to vacuum conditions. The optical system provided with thecompensation lens is calibrated under atmospheric conditions (step 514).After the optical system with the compensation lens is calibrated, thecompensation lens is removed from the optical system (step 516). Next,at least a part of the optical system is placed under vacuum conditions(step 518). Once the optical system is placed under vacuum conditions,the optical system can be used to detect sample height, inspect samplesurface, or illuminate sample.

According to the above disclosed embodiments, an optical system in acharged particle beam inspection system is assembled and calibratedunder atmospheric conditions with a removable compensation lensconfigured to compensate a drift of a focal length of a combination ofone or more lenses in the optical system from atmospheric conditions tovacuum conditions. When a part of the optical system is positioned in avacuum chamber during the operation of the charged particle beaminspection system, the drift of the focal length of the one or morelenses in the optical system has been corrected by the compensation lenswhen the optical system is assembled and calibrated. As a result, theprecision of the optical system is improved without changing theoriginal design of the optical system.

In the embodiments described in FIGS. 1-3 , charged particle beaminspection systems 100, 200, and 300 each includes a singular chargedparticle beam column 110, 210, and 310, and each one of charged particlebeam columns 110, 210, and 310 irradiates a singular charged particlebeam. However, the present disclosure is not limited thereto. That is,at least one of a charged particle beam columns 110, 210, and 310 canirradiate multiple charged particle beams. In addition, at least one ofcharged particle beam inspection systems 100, 200, and 300 can includemultiple charged particle beam columns.

The embodiments may further be described using the following clauses:

1. An optical system used in a charged particle beam inspection system,comprising:

one or more optical lenses; and

a compensation lens configured to compensate a drift of a focal lengthof a combination of the one or more optical lenses from a first mediumto a second medium.

2. The optical system of clause 1, wherein the optical system is aposition detection system comprising:

a projection module including a projection lens and configured toproject a first light beam to a sample;

a receiving module including a receiving lens and configured to receivea second light beam reflected from the sample; and

a detection module configured to detect a position of the sample basedon the second light beam.

3. The optical system of clause 2, wherein the one or more opticallenses include the projection lens included in the projection module andthe receiving lens included in the receiving module.

4. The optical system of any one of clauses 2 and 3, wherein thedetection module is disposed in the first medium, and the projectionmodule and the receiving module are disposed in the second medium.

5. The optical system of clause 1, wherein the optical system is animaging system comprising:

an illumination module configured to project a first light beam to asample;

an objective lens configured to conjugate a second light beam reflectedfrom the sample; and

a detection module configured to detect an image of the sample based onthe second light beam.

6. The optical system of clause 5, wherein the one or more opticallenses include the objective lenses.

7. The optical system of any one of clauses 5 and 6, wherein thedetection module is disposed in the first medium, and the objective lensis disposed in the second medium.

8. The optical system of clause 1, wherein the optical system is anillumination system comprising:

an illumination module configured to project a first light beam to aportion of a sample under a charged particle beam;

an objective lens configured to conjugate a second light beam reflectedfrom the sample; and

a detection module configured to detect an image of the portion of thesample based on the second light beam.

9. The optical system of clause 8, wherein the one or more opticallenses include the objective lenses.

10. The optical system of any one of clauses 8 and 9, wherein thedetection module is disposed in the first medium, and the objective lensis disposed in the second medium.

11. The optical system of any one of clauses 1 through 10, wherein thefirst medium is air, and the second medium is vacuum.

12. The optical system of any one of clauses 1 through 11, wherein thecharged particle beam inspection system is an electron beam inspectionsystem.

13. The optical system of any one of clauses 1 through 12, wherein afocal length of the compensation lens is configured based on a focallength of each one of the one or more lenses, a position of each one ofthe one or more lenses, and refractive indices of the first medium andthe second medium.14. A method for operating an optical system used in a charged particlebeam inspection system, the optical system including one or more opticallenses, the method comprising:

providing a compensation lens in the optical system, the compensationlens being configured to compensate a drift of a focal length of acombination of the one or more lenses from a first medium to a secondmedium;

assembling and calibrating the optical system in the first medium;

removing the compensation lens; and

placing the one or more optical lenses in the second medium.

15. The method of clause 14, wherein the optical system is a positiondetection system, and the method further includes:

projecting, by a projection module including a projection lens, a firstlight beam to a sample;

receiving, by a receiving module including a receiving lens, a secondlight beam reflected from the sample; and

detecting, by a detection module, a position of the sample based on thesecond light bea

16. The method of clause 15, wherein the one or more optical lensesinclude the projection lens included in the projection module and thereceiving lens included in the receiving module.

17. The method of any one of clauses 15 and 16, wherein the detectionmodule is disposed in the first medium, and the projection module andthe receiving module are disposed in the second medium.

18. The method of clause 14, wherein the optical system is an imagingsystem, and the method further comprises:

projecting, by an illumination module, a first light beam to a sample;

conjugating, by an objective lens, a second light beam reflected fromthe sample; and

detecting, by a detection module, an image of the sample based on thesecond light beam.

19. The method of clause 18, wherein the one or more optical lensesinclude the objective lenses.

20. The method of any one of clauses 18 and 19, wherein the detectionmodule is disposed in the first medium, and the objective lens isdisposed in the second medium.

21. The method of clause 14, wherein the optical system is anillumination system, and the method further comprises:

projecting, by an illumination module, a first light beam to a portionof a sample under a charged particle beam;

conjugating, by an objective lens, a second light beam reflected fromthe sample; and

detecting, by a detection module, an image of the portion of the samplebased on the second light beam.

22. The method of clause 21, wherein the one or more optical lensesinclude the objective lenses.

23. The method of any one of clauses 21 and 22, wherein the detectionmodule is disposed in the first medium, and the objective lens isdisposed in the second medium.

24. The method of any one of clauses 14 through 23, wherein the firstmedium is air, and the second medium is vacuum.

25. The method of any one of clauses 14 through 24, wherein the chargedparticle beam inspection system is an electron beam inspection system.

26. The method of any one of clauses 14 through 25, further comprising:

determining a focal length of the compensation lens based on a focallength of each one of the one or more lenses, a position of each one ofthe one or more lenses, and refractive indices of the first medium andthe second medium.

While the present invention has been described in connection withvarious embodiments, other embodiments of the invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

What is claimed is:
 1. A non-transitory computer readable medium thatstores a set of instructions that is executable by at least oneprocessor of a system to cause the system to perform a method foroperating an optical system used in a charged particle beam inspectionsystem, the optical system including one or more optical lenses, themethod comprising: compensating, by a compensation lens in the opticalsystem, a drift of a focal length of a combination of the one or morelenses from a first medium to a second medium; assembling andcalibrating the optical system in the first medium; removing thecompensation lens; and placing the one or more optical lenses in thesecond medium.
 2. The non-transitory computer readable medium of claim1, wherein the optical system is a position detection system, andwherein the set of instructions that is executable by the at least oneprocessor of the system to cause the system to further perform:projecting, by a projection module including a projection lens, a firstlight beam to a sample; receiving, by a receiving module including areceiving lens, a second light beam reflected from the sample; anddetecting, by a detection module, a position of the sample based on thesecond light beam.
 3. The non-transitory computer readable medium ofclaim 2, wherein the one or more optical lenses include the projectionlens included in the projection module and the receiving lens includedin the receiving module.
 4. The non-transitory computer readable mediumof claim 2, wherein the detection module is disposed in the firstmedium, and the projection module and the receiving module are disposedin the second medium.
 5. The non-transitory computer readable medium ofclaim 1, wherein the optical system is an imaging system, and whereinthe set of instructions that is executable by the at least one processorof the system to cause the system to further perform: projecting, by anillumination module, a first light beam to a sample; conjugating, by anobjective lens, a second light beam reflected from the sample; anddetecting, by a detection module, an image of the sample based on thesecond light beam.
 6. The non-transitory computer readable medium ofclaim 5, wherein the one or more optical lenses include the objectivelens.
 7. The non-transitory computer readable medium of claim 5, whereinthe detection module is disposed in the first medium, and the objectivelens is disposed in the second medium.
 8. The non-transitory computerreadable medium of claim 1, wherein the optical system is anillumination system, and wherein the set of instructions that isexecutable by the at least one processor of the system to cause thesystem to further perform: projecting, by an illumination module, afirst light beam to a portion of a sample under a charged particle beam;conjugating, by an objective lens, a second light beam reflected fromthe sample; and detecting, by a detection module, an image of theportion of the sample based on the second light beam.
 9. Thenon-transitory computer readable medium of claim 8, wherein the one ormore optical lenses include the objective lens.
 10. The non-transitorycomputer readable medium of claim 8, wherein the detection module isdisposed in the first medium, and the objective lens is disposed in thesecond medium.
 11. The non-transitory computer readable medium of claim1, wherein the first medium is air, and the second medium is vacuum. 12.The non-transitory computer readable medium of claim 1, wherein thecharged particle beam inspection system is an electron beam inspectionsystem.
 13. The non-transitory computer readable medium of claim 1,wherein the set of instructions that is executable by the at least oneprocessor of the system to cause the system to further perform:determining a focal length of the compensation lens based on a focallength of each one of the one or more lenses, a position of each one ofthe one or more lenses, and refractive indices of the first medium andthe second medium.
 14. An inspection system comprising: an opticalsystem including one or more optical lenses configured to: compensate,by a compensation lens in the optical system, a drift of a focal lengthof a combination of the one or more lenses from a first medium to asecond medium; assemble and calibrate the optical system in the firstmedium; remove the compensation lens; and place the one or more opticallenses in the second medium.
 15. The inspection system of claim 14,wherein the optical system is a position detection system and is furtherconfigured to: project, by a projection module including a projectionlens, a first light beam to a sample; receive, by a receiving moduleincluding a receiving lens, a second light beam reflected from thesample; and detect, by a detection module, a position of the samplebased on the second light beam.
 16. The inspection system of claim 14,wherein the optical system is an imaging system and is furtherconfigured to: project, by an illumination module, a first light beam toa sample; conjugate, by an objective lens, a second light beam reflectedfrom the sample; and detect, by a detection module, an image of thesample based on the second light beam.
 17. The inspection system ofclaim 14, wherein the optical system is an illumination system and isfurther configured to: project, by an illumination module, a first lightbeam to a portion of a sample under a charged particle beam; conjugate,by an objective lens, a second light beam reflected from the sample; anddetect, by a detection module, an image of the portion of the samplebased on the second light beam.
 18. The inspection system of claim 14,wherein the first medium is air, and the second medium is vacuum. 19.The inspection system of claim 14, wherein the inspection system is anelectron beam inspection system.
 20. The inspection system of claim 14,further configured to: determine a focal length of the compensation lensbased on a focal length of each one of the one or more lenses, aposition of each one of the one or more lenses, and refractive indicesof the first medium and the second medium.